The present invention relates to a gas turbine blade, in which a blade trunk section of a moving blade is formed having cooling passages arranged in a plurality of rows, through which a cooling medium is run in the span direction, and a blade root section is formed having a supply-side passage connected to the cooling passages so as to be able to supply the cooling medium thereto and a discharge-side passage through which the cooling medium circulated through the cooling passages is discharged.
In some of modern moving blades used in gas turbines with high turbine inlet temperature, a plurality of rows of passages are arranged in the span direction in the moving blade, and low-temperature compressed air is circulated through the passages so that moving blade can be cooled inside. According to these moving blades, which are exposed to high-temperature gas, their temperature can be restricted to a value not higher than an allowable value that is lower than a moving blade metal temperature and high enough to maintain good structural strength.
In cooling one such moving blade, cooling air supplied thereto cools it inside by convection as it passes through the cooling passages. Further, the cooling air is discharged into the high-temperature gas, which flow outside the blade, through holes in the leading edge portion, blade tip section, or trailing edge portion that is easily heated to high temperature on account of the structural conditions of the moving blade. In this manner, the edge or tip section is subjected to film cooling.
FIG. 1 is a vertical sectional view of a gas turbine blade with one such moving blade cooled by means of cooling air. As shown in FIG. 1, cooling passages 5 are formed in a blade trunk section 4 of a moving blade 1 so as to extend in the span direction between a blade root section 2 and a blade tip section 3. The passages 5 are arranged in a plurality of rows in the chord direction or the transverse direction of the blade 1, and are planted in the outer peripheral surface of the blade root section 2.
In this arrangement, cooling air 6 from an air passage in a rotor (not shown) introduced through a supply-side passage 10 in the blade root section 2 is caused to pass in the span direction between the root section 2 and the blade tip section 3, thereby subjecting the moving blade 1 to internal convection cooling.
After subjecting the moving blade 1 to the convection cooling, some of the cooling air 6 introduced through the supply-side passage 10 is discharged at high speed into a high-temperature gas 13, which flows around the blade 1, through apertures 7 that are bored through a leading edge portion 11 of the blade 1, thereby film-cooling the blade trunk section 4.
After cooling a trailing edge portion 12 of the moving blade 1 by convection, moreover, some of the cooling air 6 is discharged into the high-temperature gas 13 through holes 8 in the trailing edge portion 12 and aperture portions 9 in the blade tip section 3.
In FIG. 1, turbulators 14 are arranged in the cooling passages 5 so as to cross the flow of the cooling air 6, and serve to make the airflow turbulent, thereby improving the cooling efficiency.
Thus, in the conventional gas turbine blade, various cooling structures are used to enhance the cooling effect. Further, the structural strength is maintained by preventing intensive heating of those portions of the moving blade 1 which are thin and low in structural strength and high-temperature strength. By doing this, the operating efficiency of the operating efficiency of the moving blade 1 can prevented from lowering.
These days, moreover, a higher-temperature gas is expected to be used as an operating gas in order to improve the thermal efficiency of the gas turbine. To attain this, the material used should be higher in high-temperature strength, and the cooling effect for the moving blade must be enhanced.
Thus, a satisfactory cooling effect cannot be obtained with use of the aforesaid compressed air as the cooling medium, so that it is necessary to use steam as a cooling medium that ensures a large thermal capacity and high cooling efficiency.
With use of the gas turbine blade in which the moving blade 1 is cooled by means of the steam circulated therein, however, the thermal efficiency of the gas turbine is considerably lowered when the steam used for cooling, like the aforesaid cooling air 6, is discharged into the high-temperature gas 13. It is necessary, therefore, to recover all the steam used for cooling from the inside space of the moving blade 1 and collect the heat energy of the recovered steam by means of a steam turbine.
In consequence, in the case where the steam, like the cooling air 6, is discharged into the high-temperature gas 13, the temperature of the gas 13 is lowered considerably, and the internal efficiency of the turbine is lowered substantially. Besides, the heat energy recovered during the moving blade cooling operation cannot be utilized for the improvement of the thermal efficiency of the gas turbine. In the case where the steam is discharged into the high-temperature gas 13, therefore, the intended improvement of the thermal efficiency cannot be achieved.
Thus, the steam cooling resembles the aforesaid air cooling in that the blade root section 2 is provided with the supply-side passage through which the necessary quantity of steam for cooling is supplied to the supplied to the cooling passages 5. In the case of the steam cooling, however, low temperature steam must be fed to regions near the leading and trailing edge portions 11 and 12 of the moving blade 1 that are poor in high-temperature strength. It is necessary, therefore, to provide the supply-side passage in each of the leading and trailing edge portions 11 and 12. Moreover, the blade root section 2 should be provided with a discharge-side passage through which the steam used for cooling is discharged from the cooling passages.
In the case of the steam cooling, after all, the supply-side passage must be provided in each of those parts of the blade root section 2 which are situated close to the leading and trailing edge portions 11 and 12 of the moving blade 1, individually, in order to supply the low-temperature steam to the cooling passages in the edge portions 11 and 12. For the steam cooling, moreover, the blade root section 2 should be provided with a discharge-side passage between the supply-side passages, for discharging the steam from the cooling passage near the central portion of the moving blade 1. This discharge-side passage is used to recover the steam that is heated as it passes through the cooling passages.
Thus, for the steam cooling, the small-capacity blade root section 2 must be formed internally having the two supply-side passages, whose flow area for steam is larger than that for air, and the one discharge-side passage between them, and this is a hard task.